The invention relates to a lens antenna, preferably operable within the micro wave range, comprising a round disc-shaped lens element, such as a round disc of dielectric plastic material, having a radially varying diffraction index (dielectric constant). The lens element is positioned between two conductive plates and has feeders distributed along at least a portion of the circumference. The feeders are so shaped and oriented that they are adapted to transmit or receive a polarized wave having a polarization direction which forms an angle deviating from 90.degree., preferably 45.degree., with the major surfaces of the lens element.
Such a wave includes an E-component which is parallel with the lens plane and an E-component which is perpendicular to the lens plane. If the lens is oriented horizontally, the components are horizontal and vertical. These components are subject to diffractions and delay (phase displacement) in the lens. The dielectric constant of the disc is highest at the center and decreases with a distance from the center by a factor which is substantially proportional to the square of the normalized radial distance from the center. In order for the horizontal component to be effectively transmitted, limitations are placed on the total thickness or height of the lens, i.e. the distance between the conductive plates. However, in order to effectively transmit the vertical component, the thickness or height of the lens can be selected substantially arbitrarily. In the case of transmission of the horizontal component, cut-off appears at a lens thickness equal to .lambda./2, where .lambda. is the wave length, and the total thickness of the lens must thus exceed half the wave length at the lowest frequency in order to be able to transmit a horizontal component. There are also requirements that cross polarisation and sidelobes are suppressed. Cross polarisation is phase deviation between horizontal and vertical components which are transmitted through the same aperture. Effective cross polarization suppression requires that the horizontal and vertical components of a 45.degree. polarized wave transmitted through the lens have a phase difference which is near an integer times 2.pi. radians. Improved phase equality between horizontal and vertical components, and thereby improved cross polarization suppression, is obtained by increasing lens height. Sidelobes are caused by i.a. irregularities in the transmission phase rotation, i.e. the presence of radiation paths of different electrical lengths through the lens between its focal points and corresponding apertures. Effective sidelobe suppression requires an even phase shift across the aperture of the lens, so that the central and the peripheral rays in the lens have little phase distortion. Also, sidelobe suppression increases with lens height, because the radial distribution of the dielectric constant for vertical and horizontal E-components differ more for lenses with small height.
An increase of the lens height, however, results in a decrease of the radiation angle covered by the antenna in a plane perpendicular to the plane limiting surfaces of the lens (or the vertical plane in the given example with horizontal lens). Thus a small height is desirable for a large radiation angle. A small lens height is also desirable for preventing higher modes, which cause an unfavourable field distribution, because these increase with increasing lens height. Finally, a large lens height necessitates an increase in the plastic volume (in a lens filled with dielectric plastic material) and thereby an increased price and weight and increased space.
Thus requirement for high cross polarization suppression and high sidelobe suppression is contrary to the requirement for a large radiation angle in the plane perpendicular to the lens plane, suppression of higher modes and the minimization of weight, size and price.